One known type of information storage device is a disk drive device that uses magnetic media to store data and a magnetic head that is positioned over the magnetic media to selectively read from or write to the magnetic media.
Ordinarily, a magnetic head consists of a magnetoresistive effect (MR) read element (Reader) and an inductive write element (Writer), while some further include a heating unit (Heater). Magnetic head with a magnetoresistive (MR) read element is widely used to hard disk drive (HDD) to increase in capacity and reduce in size. Since then, several types of MR read element have been widely used by disk drive manufacturers in succession. One is anisotropic magnetoresistive (AMR) element, which makes the angle between the magnetization direction and the direction of sense current flowing through the MR element change and, in turn, cause a change the resistance of the MR element and a corresponding change in the sensed current or voltage. Another type is giant magnetoresistive (GMR) element manifesting the GMR effect. The GMR effect is a phenomenon that the magnetoresistive ratio (MR ratio) will change under an external magnetic field. The GMR element comprises two ferromagnetic layers and a non-ferromagnetic layer sandwiched between the two ferromagnetic layers. The resistance of the non-ferromagnetic layers varies with the magnetic moments of the ferromagnetic layers, the conduction electrons and the spin-dependent scattering. Still another type of MR sensor is tunnel magnetoresistive (TMR) element, which has become the mainstream MR element due to its more remarkable change of MR ratio by replacing AMR element and GMR element.
At each test after manufacturing the magnetic head, it is necessary to confirm that the finished magnetic head with an MR element is a good article and may not produce unacceptable noise. However there are many mechanisms generating noise in an MR read element, some are magnetic in nature and some are electrical in nature. The Barkhausen Noise, Random Telegraph Noise or Popcorn Noise indicates instability in an MR read element. Instability related noise tends to be a catastrophic problem rather than a contribution to background noise that can limit SNR, thus, it is necessary to handle them with different criteria respectively.
In the art today, different methods and systems are developed to measure noise in MR heads. The prevalent method is to use a digitizer to convert analog signal to digital signal, and by using a processor to analyze all original digitized signal as a whole and hence to identify whether the magnetic head produces noise. This conventional method does not handle instability related noise and background noise respectively, thus it is inadequate to identify true defects accurately. Furthermore, since the noise of a magnetic head may contain high frequency components, in order to capture such noise, the digitizer sampling frequency needs to be high enough. That increases testing cost. At the same time, the huge amount of digitized signal generated by the high speed digitizer takes a pretty long time to transfer and process the digitized signal and hence a processor with high processing power and enough memory will become essential for a faster computation. This also increases testing cost. All these are not in favor of the reduction of production costs.
In response to this, it is desired to provide a method to handle instability related noise and background noise respectively and a corresponding system to identify true defects fast and accurately to gain better cost-effectiveness to overcome the above-mentioned drawbacks.